Since aluminum nitride unites high thermal conductivity and high electrical insulating properties and is non-toxic, it is expected to be generally used in the fields enumerated above. Various kinds of aluminum nitride sintered bodies for use in these applications and processes for the production thereof have hence been proposed.
First, a process for producing an aluminum nitride sintered body having a surface capable of being directly metallized was disclosed in JP-B-3-13190 (the term "JP-B" as used herein means an "examined Japanese patent publication."), which process comprises adding 0.1 to 10 parts by weight of a Group 4a, 5a, or 6a element, e.g., Ti, to 100 parts by weight of aluminum nitride and sintering the mixture at ordinary pressure in a non-oxidizing atmosphere having a boron source. In JP-A-2-124772 (the term "JP-A" as used herein means an "unexamined published Japanese patent application.") was thereafter disclosed a process for producing a black aluminum nitride sintered body having a thermal conductivity of 150 W/m.multidot.K or higher which comprises preparing an aluminum nitride powder regulated to have an oxygen content of 0.5 to 2.5 wt % as the main ingredient, adding 0.01 to 1.0 wt % TiO.sub.2 in terms of the amount of Ti and 0.01 to 1.0 wt % Y.sub.2 O.sub.3 as a sintering aid to the aluminum nitride powder, and sintering the mixture in a non-oxidizing atmosphere.
The present inventors also disclosed in JP-A-3-290371 a process for producing a black aluminum nitride sintered body having no irregular color which comprises mixing 100 parts by weight of aluminum nitride with 0.1 to 10 parts by weight of TiO.sub.2 as a colorant and a small amount of Y.sub.2 O.sub.3 as a sintering aid, forming the mixture to prepare a green body, calcining the formed green body, and then sintering the calcined body in a non-oxidizing atmosphere.
However, in JP-B-3-13190 and JP-A-2-124772, optimum conditions for producing a black sintered body having no irregular color are not disclosed. Hence, when conditions within the ranges disclosed in these references are used to produce sintered bodies, there are cases where the sintered bodies have irregular color, resulting in a low yield because of the appearance defect. JP-A-3-290371 also failed to elucidate the optimum amount of the sintering aid and appropriate conditions for calcining. Because of this, there are cases where the sintered bodies produced by this prior art technique have been insufficiently densified or suffer bleeding of the sintering aid, although they are black.